Aluminoxanes, made by the incomplete hydrolysis of hydrocarbyl aluminums, find extensive use as alkyl substitutes in Ziegler-Natta catalyst systems and as cocatalysts for ethylene trimerization.
There are two kinds of aluminoxanes: cyclic aluminoxanes, which can be represented by the formula --[R--Al--O]--.sub.n, and linear aluminoxanes, which can be represented by the formula R--[R--Al--O]--.sub.n AlR.sub.2.
Synthesis of the cyclic aluminoxane can be described by the following equation: EQU nR.sub.3 Al+nH.sub.2 O.fwdarw.(RAlO).sub.n +2nRH
A typical hydrolysis can be carried out at atmospheric pressure at a temperature in the range of about 0.degree. C. to about 100.degree. C., and preferably at a temperature in the range of about 5.degree. C. to about 15.degree. C. Water is added to a solution of, for example, trialkyl aluminum in an anhydrous, inert organic solvent. The concentration varies from about 5 percent by weight aluminum compound to about 75 percent by weight based on the total weight of the solution. The water is preferably added slowly but in a single batch, with vigorous stirring and cooling. The reaction is considered complete when effervescence ceases.
Another method for preparation of the aluminoxane is accomplished by using the water of hydration of metal salts, e.g. by adding about 0.1 to about 0.16 mole of solid magnesium sulfate heptahydrate to a ten percent solution of triisobutyl aluminum in heptane. The mixture is stirred vigorously until effervescence ceases, usually overnight or even longer.
The solutions are stored under an inert gas such as argon.
Examples of suitable solvents are heptane, hexane, pentane, isooctane, purified kerosene, cyclopentane, cyclohexane, methylcyclopentane, and dimethylcyclopentane. The use of 1-hexene is found to be advantageous in some cases. Benzene, toluene, and xylene can be used, but are not preferred.
Examples of useful hydrocarbyl aluminum compounds are as follows: tri-isobutylaluminum, trihexyl aluminum, isobutyl aluminum dihydride, hexyl aluminum dihydride; di-isobutyl aluminum hydride, dihexyl aluminum hydride, di-isobutylhexyl aluminum, isobutyl dihexylaluminum, trimethyaluminum, triethylaluminum, tripropylaluminum, tri-isopropylaluminum, tri-n-butylaluminum, trioctylaluminum, tridecylaluminum, tridodecylaluminum, tribenzylaluminum, triphenylaluminum, trinaphthylaluminum, and tritolylaluminum. The preferred hydrocarbyl aluminums are tri-isobutylaluminum, trihexyl aluminum, di-isobutyl aluminum hydride, and dihexyl aluminum hydride.
The techniques for the preparation of aluminoxanes should be chosen to ensure that hydrolysis is both uniform and incomplete, the complete hydrolysis product being hydrated aluminum oxide. The undesirable complete hydrolysis is usually the result of localized high water concentrations, typically at water to aluminum mole ratios of about 1.5.
Unfortunately, the prior art methods suffer from these localized high water concentrations, ill-defined hydrolysis ratios so important to ethylene trimerization, resistance to scale-up, and relatively long preparation times, and the problem of waste streams or recycle streams where hydrated salts are concerned.